Femoroacetabular impingement (FAI) is a common mechanism leading to hip injury, leading to hip pain and early osteoarthritis (OA). While a benefit to FAI surgery has been suggested, no studies have identified prognostic factors associated with successful FAI arthroscopy, leaving surgeons without adequate indications. Despite this lack of evidence, rapid increase in the use of hip arthroscopy is reminiscent of that seen for knee and shoulder arthroscopy. The enthusiasm for hip arthroscopy has clearly grown at a faster rate than our ability to accumulate information. The patient population currently treated operatively for FAI is highly heterogeneous, making a randomized controlled trial (RCT) extremely difficult to mount. This observational study will establish clinical equipoise for a futue RCT. Aims/Methods: (Aim 1): Determine the clinical and morphologic factors associated with successful FAI management. Success will be defined by an improvement in pain and function measured using a hip-specific patient-reported outcome instrument 2 years post-presentation. Operative and non-operative patients will be propensity score matched to limit the study to those eligible for either therapeutic approach. Patient characteristics and bony morphology will be assessed with regard to the development of hip pain and early chondro-labral pathology and their effect on patient outcome at 2 years. (Aim 2). Measure the effect of FAI arthroscopy on subsequent cartilage degeneration. A random sample of patients who had initial quantitative MRI assessment (T2 mapping and T1 rho) will be followed up with MRI 5 years after surgery to assess cartilage deterioration. Accomplishing these aims will allow us to test 3 hypotheses: Hypothesis 1A. Patients undergoing FAI arthroscopy for certain indications (e.g., cam lesion) will have improved function at 2 years compared to patients who choose non-operative management. Hypothesis 1B. Arthroscopic correction of abnormal bony morphology associated with intra-articular derangement will result in improved patient outcomes compared to surgical interventions that do not address bony morphology. Hypothesis 2. Progression of cartilage degeneration will be less pronounced among patients undergoing FAI arthroscopy for bony morphology correction than those treated non-operatively for similar morphology. Benefits: The first aim will clarify prognostic factors associated with improved outcomes and will provide surgeons with evidence for appropriate indications for FAI treatment while the second aim will provide the first evidence that appropriately identified patients have a decreased risk of early OA. Achieving these aims, we will identify appropriate indications, prognostic factors, and treatment algorithms for FAI. We will also have identified a more homogeneous subset of patients for which an RCT will determine the best course of treatment. The use of propensity score matching in the setting of arthroscopic surgery will also aid other researchers struggling with the issues of generalizability and timely completion of RCTs.